(Not applicable)
(Not applicable)
This invention relates generally to ring laser gyroscopes and more particularly, to multioscillator ring laser gyroscopes.
The ring laser gyroscope (RLG), in its simplest form, is a device comprising an arrangement of mirrors for directing light beams around a closed path through a gain region comprising a lasing gas and an arrangement of electrodes for creating an electrical discharge in the gas and a means for measuring the frequency difference of light beams thereby generated that are propagated around the closed path in opposite directions. The frequency difference of the light beams is a measure of the rotational rate of the RLG apparatus in the plane of the light beams.
A serious problem with this two-frequency RLG is that rotational rates near zero are difficult to measure because of lock-inxe2x80x94the coupling of the counter-propagating light beams as a result of backscatter arising from non-ideal optics. A commercially-successful two-frequency RLG has evolved that circumvents the lock-in problem by separating the frequencies of the counter propagating light beams at zero rotation rate by creating an artificial rotation rate. This artificial rotation rate is brought about by mechanically dithering either the RLG block or a mirror.
The multioscillator RLG represents a more sophisticated approach to solving the lock-in problem by utilizing a purely optical scheme. The scheme is based on the establishment of four resonant modes for the mirror system by placing, for example, a reciprocal polarization rotator and a nonreciprocal polarization rotator in the light path. The lock-in problem is avoided since the four resonant frequencies associated with the four resonant modes are all different, even at a zero rotation rate.
A typical resonant mirror system for a multioscillator RLG is shown in FIG. 1. The four mirrors 1 constrain resonant light beams traveling in opposite directions to light path 3. Circularly-polarized light beams experience reciprocal polarization rotations in reciprocal polarization rotator 5 and non-reciprocal polarization rotations in the Faraday rotator 7. The magnetic field required by the Faraday rotator 7 is provided by permanent magnets 9 with magnetic fields within the magnets having directions as shown by the arrows.
The four resonant modes are CW/LCP, CCW/LCP, CCW/RCP, and CW/RCP, the acronyms CW and CCW standing respectively for clockwise and counterclockwise propagation around the closed path and LCP and RCP standing respectively for left-circularly-polarized light and right-circularly-polarized light. A measure of the rotation rate is obtained by first taking the differences in the frequencies of the right-circularly-polarized light beams and the frequencies of the left-circularly-polarized light beams and then taking the difference in the differences.
A typical example of a reciprocal polarization rotator is a crystalline-quartz element with its optic axis aligned with one portion of the light-beam path. Another way of achieving reciprocal polarization rotation is by using a non-planar light-beam path geometry. The non-reciprocal polarization rotator is typically a Faraday rotator consisting of a thin glass disc in which there is a magnetic field normal to the disc.
Another characteristic of modem RLGs is the use of some means of focusing the light beams so as to minimize the light-beam dimensions transverse to the light path. The usual focusing approach is to utilize a curved mirror for at least one of the mirrors that direct the light beams around the closed path.
The frequency of each of the four resonant laser modes of a multi-oscillator RLG is such that an integral number of wavelengths will fit exactly within the path length of the resonant cavity. A gas mixture, typically comprised of helium and neon, provides gain for the laser beam. In order to ensure proper laser operation, the cavity length must be tuned in such a way that the gas medium will supply sufficient gain at the cavity""s resonant frequencies. The methods of ring laser gyro cavity length control are extensively discussed in the literature.
In a multioscillator RLG, four laser modes with widely separated frequencies must be simultaneously sustained within the cavity. A gas mixture providing gain over a wide range of frequencies is used to ensure sufficient gain for all of the modes when the cavity is properly tuned using, for example, the methods disclosed in U.S. Pat. No. 5,208,653.
Cavity length control is usually accomplished by observing the laser intensity and controlling the voltage applied to a piezo-electric transducer which in turn applies force to a mirror diaphragm. This force causes a very slight motion of the mirror face, thereby changing the cavity length. A servo is used to apply the voltage which maximizes the laser intensity.
It has been observed that under high-shock accelerations, the mirror and piezo-electric forcer assembly also move relative to the cavity body due to their inertia and the finite stiffniess of the assembly and the servo loop. As a result, the instantaneous cavity length can change substantially during the shock. If the length change is large enough (more than a third or half a laser-mode wavelength of 630 nm). some of the laser modes may drop out due to insufficient gain. This radically affects the operation of the gyro and can cause erroneous angle indications.
This invention takes advantage of certain properties of the multioscillator RLG to provide accurate gyro outputs even under large-shock conditions. As shown in FIG. 2a, the multioscillator RLG operates, in the absence of the reciprocal and nonreciprocal polarization rotators 5 and 7 of FIG. 1, at a single frequency denoted by the intensity arrow near the maximum of the gain curve. The reciprocal polarization rotator 5 splits this single mode into RCP and LCP modes as shown in FIG. 2b. With this mode splitting the multioscillator RLG becomes two circularly-polarized gyros, an RCP gyro and an LCP gyro, co-existing in the same cavity. The resonant frequencies of the two gyros are sufficiently far apart as to be separable, and the gas mixture is chosen to ensure a broad gain curve which allows both modes to lase simultaneously.
The nonreciprocal polarization rotator 7 splits the RCP and LCP modes into CW and CCW modes as shown in FIG. 2c. The mode frequencies in the absence of any angular rotation of the RLG (FIG. 2c and FIG. 2d) are shifted by an angular rotation of the RLG as shown in FIG. 2e. Each of the two gyros is biased by a magneto-optic Faraday crystal in order to provide a separation between the clockwise and counter-clockwise beams and thereby prevent lock-in. The Faraday bias corresponds to a large angular rotation rate (on the order of 1000 degrees/sec) and is sensitive to temperature, magnet field strength, etc.
The two-gyro configuration of the multioscillator RLG permits common mode cancellation of the Faraday bias while providing an accurate angular rotation measurement. This is accomplished in the following way. The output frequencies fR and fL of the RCP and LCP gyros is the difference in frequency of the CCW and CW modes. Thus,                               f          R                =                              f            F                    -                                    1              2                        ⁢                          f                              Δ                ⁢                                  xe2x80x83                                ⁢                0                                                                        (        1        )                                          f          L                =                              f            F                    +                                    1              2                        ⁢                          f                              Δ                ⁢                                  xe2x80x83                                ⁢                θ                                                                        (        2        )            
where fF is the Faraday bias frequency and fxcex94xcex8 is proportional to the angular rotation measure of the RLG. For convenience, we will refer to fxcex94xcex8 as the xe2x80x9cangular rotation measurexe2x80x9d.
In normal operation, the angular rotation measure is determined by taking the difference of the two gyro output frequencies:
fxcex94xcex8=fLxe2x88x92fRxe2x80x83xe2x80x83(3)
The Faraday bias frequency can be determined from any of the equations                               f          F                =                              f            L                    -                                    1              2                        ⁢                          f                              Δ                ⁢                                  xe2x80x83                                ⁢                θ                                                                        (        4        )                                          f          F                =                              f            R                    +                                    1              2                        ⁢                          f                              Δ                ⁢                                  xe2x80x83                                ⁢                θ                                                                        (        5        )                                          f          F                =                              1            2                    ⁢                      (                                          f                L                            +                              f                R                                      )                                              (        6        )            
In the event of a shock or other disturbance which causes a shift in the resonant frequencies with respect to the gain curve, one of the two gyros may operate marginally due to lack of gain to the point where its frequency cannot be detected. In this situation, the calculated fxcex94xcex8 will be invalid due to an invalid LCP or RCP gyro output. However, it should be noted that at least one of the two gyros will always remain operational as a shift in the gain curve leading to decreased gain in one gyro will actually lead to a gain increase for the other gyro. This invention is a method of using a single gyro within the multioscillator RLG to provide rate information for a short period of time where the second gyro in the multioscillator RLG is inoperative.
The invention is a method and apparatus for processing signals with frequencies fL and fR from a multioscillator ring laser gyro, the method being repeated at regular time intervals. The difference fxcex94xcex8, of fL and fR is a measure of the angular rotation rate of the ring laser gyro and the sum fF of fL and fR divided by 2 is the Faraday bias frequency.
The first step of the method comprises determining two or more of the values MLP, MRP, MFP, and Mxcex94xcex8P of a set of functions ML(fL), MR(fR), MF(fF), and Mxcex94xcex8(fxcex94xcex8). The second step comprises storing two or more processed values MLS, MRS, MFS, and Mxcex94xcex8S of the functions ML(fL), MR(fR), MF(fF), and Mxcex94xcex8(fxcex94xcex8) if the corresponding values of MLP, MRP, MFP, and Mxcex94xcex8P are valid. A processed value is derived from the value for the present time interval and zero or more processed values for prior time intervals.
The third step comprises determining fxcex94xcex8P or a function thereof from zero or more valid function values obtained as a result of executing the first step during the current time interval, zero or more processed values obtained as a result of executing the second step during a prior time interval, and zero or more extrapolated values obtained by extrapolating the processed values obtained as a result of executing the second step during a prior time interval to the present time interval.
A function value is concluded to be valid if a rule is satisfied, the rule being selected from the group consisting of (1) the measure of at least one component of the acceleration of the ring laser gyro is less than a specified positive threshold and (2) the measure of at least one component of the acceleration of the ring laser gyro is greater than a specified negative threshold.
An alternative basis for concluding that a function value is valid is if an associated validity index does not exceed a threshold value, a validity index being a measure of (1) the difference in the present function value and the processed function value determined in a prior time period or (2) the difference in the present function value and the processed function value determined in a prior time period and extrapolated to the present time interval.